If you buy apps from the Mac App Store, then getting updates is fairly easy. (For all of its problems, the Mac App Store does updates pretty well.)
However, there are a lot of good apps -- maybe even the majority of good apps -- which aren’t available in the Mac App Store.
going to Google to find the appropriate website (not always easy because, especially if app has a common name, or if the search results have a lot of other download sites before the official site.)
Most apps have an update system, but they usually want to update at the least convenient time: right after you launch them, when you are trying to use them for something.
Wouldn’t it be better if you could check for updates right after an app quits? Or check in the middle of the night or when your computer isn’t in use?
(Note: Technically these scripts do three things: Download, Install, and Update; but I didn’t want to prefix them with dui and diu seemed awkward to type, so I went with a di prefix.)